Such swings are a characteristic—but previously neglected—impact of commercial fishing, according to researchers who conducted the first systematic comparison of population changes in harvested and nonharvested fish species.

Fisheries managers have long encouraged the industry to target more mature fish, on the assumption that the younger generations left behind would thrive and ensure a stable population.

But big fish, the study authors say, may help buffer populations from the effects of random environmental variation, such as disease, pollution, or shifting water temperatures (related news: "Lice From Fish Farms Killing Wild Salmon" [October 2, 2006]).

Larger individuals are "better able to withstand lean times and are responsible for the lion's share of reproduction," said study co-author George Sugihara of the Scripps Institution of Oceanography in San Diego, California.

He says that stocks in which the big fish have all been caught face a kind of double jeopardy: The risk posed by long-term declines is magnified by erratic, shorter-term fluctuations.

The study, co-led by Sugihara and Scripps colleague Chih-hao Hsieh, implies that some overfished populations may be closer to collapse than had previously been thought.

But the finding also points to a new way of diagnosing problems in fisheries before it is too late.

"You might be able to use change in age structure and increase in variability as an early warning sign that a fishery is in trouble," Sugihara said.

Half a Century of Data

For years researchers have been struggling to understand exactly how commercial fishing affects fish populations.